1. Field of the Invention
The present invention relates to a communication system connected to a telephone line, and more particularly to a technique for amplifying a signal on a telephone line according to a loop current detected from the telephone line.
2. Description of the Related Art
In general, communication systems such as a telephone, a facsimile machine, a digital telephone answering device (TAD), a digital speakerphone, etc. are connected to a telephone line. Further, it is well known that a signal intensity at the terminals connected to the telephone line is dependent on the distance from a switching system. Since an impedance of the common telephone line made of copper varies according to the length thereof, the relative current with respect to a constant DC current, i.e., the loop current, also varies according to the distance between the communication system and the switching system. The variation of the loop current corresponds to a variation of the distance. From this, it can be understood that a variation of the signal intensity according to the distance from the switching system is proportional to the variation of the loop current. Therefore, a user of the telephone distanced far away from the switching system can hardly hear the called party. In order to solve such problem, it is necessary to compensate for an attenuation of the signal on the telephone line.
The Feiner et al., Terry, Martin, and Eberhardt et al. patents, U.S. Pat. Nos. 4,228,323, 4,453,037, 4,002,852, and 4,453,038, entitled respectively Variable Loop Length Compensated Barrier Circuit, Loop Length Compensation Circuit, Electronic Telephone Network, and Circuit Arrangement For Producing Control Means From Line Current, each disclose telephone systems in which the amplification is varied according to the measured loop current. However, none of these patents teaches or suggests the use of a switching table as in the present invention.
The following additional patents each disclose features in common with the present invention but are not as pertinent as the patents discussed above: U.S. Pat. No. 3,903,378 to Lee et al, entitled Arrangement For Controlling The Gain Of Two-Way Amplifiers In Accordance With Loop Lengths, U.S. Pat. No. 5,321,746 to Bader, entitled Adjustable Gain Range Current Mirror, and U.S. Pat. No. 4,720,856 to Pace et al, entitled Control Circuit Having A Direct Current Control Loop For Controlling The Gain Of An Attenuator.